1. Field of the Invention
The present invention relates to a trench gate semiconductor device having a gate electrode in a trench formed in a semiconductor region and a method of manufacturing the same, and in particular, to a trench gate semiconductor device in which a Schottky diode is integrally formed in parallel between a source and a drain and a method of manufacturing the same.
2. Description of the Related Art
In a DC-DC converter for voltage step-down, a MOS transistor is used as a switching (chopping) element. In a period during which current passage from a primary side to a secondary side is interrupted by the switching, a reflux structure by a flywheel diode can be useful to prevent the interruption of the current on a load side. However, in accordance with an increasing demand for lower voltage as output voltage of the load side, forward voltage drop of the aforesaid diode cannot be neglected as a loss. A structure used to solve this problem is to provide another MOS transistor (second MOS transistor) instead of the diode, and to turn on this MOS transistor between its source and its drain in the same period as a conduction period of the diode. A so-called trench gate MOS transistor having a trench gate structure is one of MOS transistors for such use.
In the above-described structure, it is difficult to control gate voltage so as to turn on the second MOS transistor in exactly the same period as the conduction period of the diode, and in actual practice, a period when the chopping MOS transistor and the second MOS transistor both turn off (dead time) is provided. In this dead time, the second MOS transistor functions as a diode (pn junction diode) that the second MOS transistor has as its parasitic element. This period, even though short, also involves a problem of forward voltage drop as a loss. As a solution, there has come into use a MOS transistor in which a Schottky diode is internally formed in parallel in the second MOS transistor so that the forward voltage drop in the dead time can be reduced.
In the MOS transistor having the built-in Schottky diode, it is necessary to be careful about breakdown tolerance as one of its ratings when backward voltage is applied to the Schottky diode (and when the drain-source is off). In the MOS transistor having the Schottky diode, when the backward voltage is applied, breakdown generally occurs in the Schottky diode portion first at voltage lower than voltage at which breakdown occurs in the MOS transistor portion, due to the property of the element. It is preferable that current allowed to pass at this time is larger since this means that the breakdown tolerance is ensured. However, the Schottky diode is an additionally formed diode and it generally occupies a small area. Therefore, the tolerable breakdown current is lower than that of a MOS transistor having no built-in Schottky diode.
An example of a MOS transistor having a built-in Schottky diode is disclosed in the following JP-A 11-154748(KOKAI) (see, for example, FIG. 6 in the same document).
[Reference 1] JP-A 11-154748(KOKAI)